Mishnah Niddah 3:2: If she miscarries something like a membrane, like a hair, like dust, like red flies, she should put it in water, [and] if it dissolves she is impure; and if not, she is pure. If she miscarries something like fish, [like] locusts, [like] shekatsim u'remasim [small animals, such as reptiles, insect, or rodents, which move by scurrying, creeping, slithering, etc.], if they had blood with them, she is impure; and if not, she is pure. If she miscarries something like a [domesticated] animal, a beast, or a fowl, whether pure or impure ones [i.e. kosher or not kosher], if it is male, she should sit for [the number of postpartum days of impurity and purity required for one who gives birth to] a male; and if it is female, she should sit for [the number of postpartum days of impurity and purity required for one who gives birth to] a female. And if [its sex is] unknown, she should sit for [the number of days of impurity and purity required for one who gives birth to] a male and for [one who gives birth to] a female [i.e. she should be stringent in both directions, with a longer period of initial impurity, and a shorter period of subsequent purity], according to Rabbi Meir. And the Sages say: anything that does not have a human shape is not [considered to be] a fetus [with regard to the purity status of one who gives birth].

הַמַּפֶּלֶת כְּמִין קְלִפָּה, כְּמִין שַׂעֲרָה, כְּמִין עָפָר, כְּמִין יַבְחוּשִׁים אֲדֻמִּים, תַּטִּיל לַמַּיִם. אִם נִמֹּחוּ, טְמֵאָה. וְאִם לָאו, טְהוֹרָה. הַמַּפֶּלֶת כְּמִין דָּגִים, חֲגָבִים, שְׁקָצִים וּרְמָשִׂים, אִם יֵשׁ עִמָּהֶם דָּם, טְמֵאָה. וְאִם לָאו, טְהוֹרָה. הַמַּפֶּלֶת מִין בְּהֵמָה, חַיָּה וָעוֹף, בֵּין טְמֵאִין בֵּין טְהוֹרִים, אִם זָכָר, תֵּשֵׁב לְזָכָר. וְאִם נְקֵבָה, תֵּשֵׁב לִנְקֵבָה. וְאִם אֵין יָדוּעַ, תֵּשֵׁב לְזָכָר וְלִנְקֵבָה, דִּבְרֵי רַבִּי מֵאִיר. וַחֲכָמִים אוֹמְרִים, כֹּל שֶׁאֵין בּוֹ מִצּוּרַת אָדָם, אֵינוֹ וָלָד:

Mishnah Niddah 3:3: If she miscarries a sac full of water, full of blood, or full of strands [of flesh], she need not be suspicious that it was a fetus. But if its was assembled [into a human form], she should sit for [the number of postpartum days of impurity and purity required for one who gives birth to] a male and for [one who gives birth to] a female.

הַמַּפֶּלֶת שְׁפִיר מָלֵא מַיִם, מָלֵא דָם, מָלֵא גְנוּנִים, אֵינָהּ חוֹשֶׁשֶׁת לְוָלָד. וְאִם הָיָה מְרֻקָּם, תֵּשֵׁב לְזָכָר וְלִנְקֵבָה: